Typical solvent-based coating compositions contain 40-50 wt. % volatile organic solvents as the viscosity reducer, while typical water-based coating compositions contain 25-35 wt. % water and 10-20 wt. % volatile organic solvents as the viscosity reducers. The present coating compositions utilize reactive polyester polyols and reactive diluents as viscosity reducers. These viscosity reducers are substantially non-volatile at the coating composition cure temperatures conventionally employed during coating operations and may partially co-react with the remainder of the coating composition.
There is a need for a coating composition for the coating of metal substrates which possesses the requisite properties of rapid drying, high total solids content and low viscosity range while at the same time having a relatively low VOC content. Coating compositions having high VOC contents due to highly volatile oxygenated solvents such as glycol ethers, ketones, esters and the like are undesirable since they present safety and health hazards (fires, explosions and inhalation by workers of the large quantities of volatile solvents stored on the premises). Moreover, such solvents also present a serious environmental risk to air quality and coating plants must contain equipment for trapping and incinerating fugitive solvent emissions. Volatile solvents are also undesirable from a product point of view since they leave bubbles or cracks in the coating as they evaporate or they may be trapped between the outer surface of the base coating and the overprint varnish which is typically added after the substrate has been coated and printed.
Thermosetting compositions useful as a coating or ink and containing a thermosettable polyester, aminoplast resin, reactive diluent and alkali metal organosulfonate as a catalyst for curing the compositions are well known in the prior art, see U.S. Pat. No. 5,068,273 issued Nov. 26, 1991 to the present assignee. However, the compositions of the '273 patent do not meet the requisite properties of the present compositions.